Debris in field

Tail piece

A piece of Flight MH17's tail, with the Malaysia Airlines marking, is seen lying on its own about 100m (330ft) south of the other debris.

Three teams of international experts have now reached the wreckage of the Malaysia Airlines plane that crashed in eastern Ukraine on 17 July. World leaders have called for the investigation to start urgently but there are fears that the crash site may have been compromised. Rob Brown and Alison Trowsdale look into what challenges the air crash investigators will face when they start to piece together the final moments of flight MH17.

The crash site

Could the evidence at the crash site of flight MH17 have been tampered with or contaminated? Footage has shown cranes lifting large sections of the fuselage. A video which emerged on Sunday showed pro-Russian rebels shortly after the crash searching through wreckage and the personal belongings of those on board.

Monitors from the Organisation for Security and Co-operation in Europe (OSCE) have visited the site several times.

Their spokesman Michael Bociurkiw told the BBC World Service that major pieces of the aircraft - including the tail fin - looked different during their most recent visit from when they first saw them. He said the pieces looked as though they had "been cut into".

Aviation expert David Learmount says the usual procedure is for a crash site to be secured like a crime scene so that evidence is not disturbed until official investigators arrive.

"This particular site has never been secured and it still isn't," he says.

The pattern of the wreckage on the ground is also important. If it is scattered across an extremely wide area, then it shows that the plane "blew up in the sky and fell apart in the sky," says Mr Learmount. Moving or tampering with the wreckage would disrupt that pattern.

How a missile could have brought down MH17

The wreckage

Once at the crash site, investigators will want to look at the external marks on the wreckage, says David Gleave, an aviation expert from Loughborough University and a former air crash investigator.

"Some pictures I've seen look as if various panels of the plane have been hit with a 12-bore shotgun and that shows evidence of something from the outside trying to get in, rather than a blast inside the aircraft trying to get out."

Douglas Barrie, a senior fellow at the International Institute for Strategic Studies (IISS) says: "The imagery that has so far been available is certainly in keeping with the kind of damage one would expect from a surface-to-air missile with a fragmentation warhead."

There has been speculation that a Buk or SA-11 missile was responsible - this type of missile is equipped with a proximity fuse which can sense when the target is close and detonate the missile before impact, firing out fragments of shrapnel.

But David Owen, author of the book Air Accident Investigation, says, "Even if fragments of a missile were retrieved, it would not be enough to establish responsibility because both sides have access to this kind of equipment."

But with parts of the aircraft already moved, how difficult will it be for investigators to prove the cause of the crash?

Mr Gleave says it would be incredibly difficult to substitute the aircraft parts or to cover up the damage.

"Instant and secure access to a crash site is always ideal," he says, but he believes that if the crash was indeed caused by a missile, there will be enough evidence on the ground.

The outside skin of the aircraft also can provide essential evidence, says Mr Learmount. "It could contain traces of explosives from an internal blast or a missile - unless it has been tampered with," he says.

"In theory, air crash investigation techniques are very good but the big problem here is possible crime scene contamination," says Mr Owen. "Bodies have been moved, pieces of wreckage have been moved and there are the chaotic conditions of a war zone - all that means information may be obscured or the wrong conclusions may be drawn from it."

The flight data recorder (FDR) is designed to record the operating information from the plane's systems. Whenever the pilot touches the controls or changes course, the FDR records that action.

The cockpit voice recorder (CVR), as its name suggests, records conversations between crew members on the flight deck and any other sounds that occur within the cockpit. In commercial aircraft there are usually several microphones sending audio information to the CVR.

There were two black box recorders on flight MH17, storing key information about the flight as well as conversations in the cockpit. The pro-Russian rebels in eastern Ukraine handed over both recorders to Malaysian experts on Monday and the head of the Malaysian delegation said they were "in good condition." But could the information inside have been tampered with?

"It is possible to tamper with black boxes, but in order to be successful, it takes a lot longer than four days," says Mr Gleave. "Someone could have interfered with the recorders to make the data unreadable but in that short space of time, you couldn't substitute fake data for real data to come up with a different cause of accident."

Robert Francis, former vice-chairman of the US National Transportation Safety Board (NTSB), agrees. He says flight recorders can be damaged, rendering them unreadable, but he has not heard a case of the data inside being tampered with. The chance of the recorders not containing good data is very slim, he says.

Image copyrightAFPImage caption
Rebels allowed the train carrying the bodies to leave their territory after negotiations with the Malaysians

The victims

The bodies of those killed in the crash have been taken out of rebel-controlled territory and will be flown to the Netherlands for identification. However, Dutch Prime Minister Mark Rutte has warned that this process could take months.

There are also concerns how the bodies have been treated, stored and transported.

Mr Francis says that, depending on how much has been taken from the crash site, there will be big challenges for any scientific investigation. "Just the fact that all the victims are gone makes it difficult to do any kind of analysis on how the aircraft blew apart and how the passengers died."

"The bodies of those who died need to go under pathological examination," says Mr Gleave. This will allow investigators look at shrapnel wounds and will assist with information on whether it was a bomb onboard the plane or an external explosion coming into the cabin.

Some experts are warning that even talk of evidence tampering might bring the results of any investigation into question. "Anybody who doesn't like the outcome of this investigation can claim it is not valid because nothing has been protected," says Mr Learmount.

Michael Bociurkiw, David Owen, Robert Francis and David Learmount spoke to the BBC World Service